1. Field of the Invention
The present invention relates to tubular structures, such as vascular grafts and the like, that are adapted to carry a fluid stream, and particularly to tubular structures that are resistant to leakage following a puncture.
2. Description of Related Art
Tubular structures are used to carry a variety of fluid streams, including both liquids and gases. It is desirable for many of these tubular structures to be resistant to punctures, either accidental or intentional, and/or to provide some degree of self-healing following a puncture so as to minimize fluid leakage.
Of particular interest to the present inventors is the possibility of creating a tubular structure for carrying blood or other body fluids that will provide improved self-healing characteristics. For instance, in the instance of vascular grafts, these devices often must be punctured both in initial installation (for example, while being sutured in place) and in subsequent medical procedures (for example, to provide an injection, to place a catheter, or the like). As a result, it is highly desirable that blood loss is minimized after each puncture through such tubes.
Even more critical is when vascular grafts are used as an access site for dialysis and the like. In the case of an arterial-venous ("AV") access graft, these vascular grafts must withstand several punctures a week over the life of the graft. This is a highly demanding environment that inevitably leads to graft failures due to excessive numbers of puncture holes, often requiring installation of two or more grafts over the course of a year. Unfortunately, each new AV access graft generally will not function properly until it has achieved tissue in-growth in a patient, a process that can take weeks after installation to occur adequately. Puncture of these tubes ("cannulation") before full tissue in-growth may result in excessive subcutaneous bleeding and serious complications.
A number of solutions have been proposed to reduce tubular leakage following punctures. For example, in U.S. Pat. 4,184,489 to Martinez it is proposed to use two concentric elastomeric sleeves, with the outer sleeve sized to radially compress the inner sleeve. This construction is reported to improve the sealing of the device following puncture. Similarly, in U.S. Pat. 4,184,489 to Burd it is taught to surround a blood conduit tube with an elastomeric sleeve and then a C-shaped needle-impenetrable member partially surrounding the sleeve. This device is again asserted to prevent leakage into and out of the tube during and after needle penetration by maintaining a radial compression on the tube.
A number of other devices teach employing two or more layers of different materials to achieve some degree of puncture hole contraction following removal of the puncture device. For example, in U.S. Pat. No. 4,619,641 to Schanzer it is proposed to employ two concentric expanded polytetrafluoroethylene (PTFE) tubes having a gap between them and then filling the gap with a silicone adhesive layer. Comparing the performance of this device with a device having the same two expanded PTFE tubes and no silicone layer filling the gap between them, Schanzer reports significantly reduced blood loss with the three layered PTFE-silicone-PTFE hemoaccess tube.
While these various devices may provide some improvement in leakage protection over puncturing through a single layer tube, they work on a similar principle that the puncture hole should be constrained with an elastomer or similar material so that it will more quickly contract to reduce leakage upon removal of a needle. There are a number of serious drawbacks with this approach. First, these elastomer tubes tend to have poor handling characteristics, with some surgeons referring to them as "rubber hoses" because they bend and handle so poorly. Second, these tubes generally do not sew in place well, again making them difficult for surgeons to handle and install. Third, these tubes tend to be difficult to "revise" if a blockage occurs in them. Finally, since these tubes rely only on the elastic qualities of the tube wall to seal holes, there will always remain a hole all the way through the wall of the device following needle removal. Even with rapid contraction around the hole, some leakage through the hole after needle removal will be expected to occur. Additionally, the presence of a continuous hole through the wall of the device also is believed to compromise device integrity over time.
Another problem with multiple layered devices is that size is often a constraint in creating any implantable device. Most implantable devices must be as small as possible since space is almost always quite limited. Unfortunately, most of the devices that reduce leakage rely on an increased in the bulk of wall layers to accomplish rapid hole reduction (that is, by increasing the thickness of the wall layers by laminating together multiple layers or simply using thicker wall material). This inevitably leads to a compromise between effective leakage reduction and the total acceptable size of the device.
Finally, many of these previous devices require use of less than preferred implantable materials. While PTFE is a preferred artificial material for implantation, since it is highly bio-compatible, it has rather poor recovery properties following puncture. As a result, prior devices have employed other materials to aid in quickly reducing hole size, such as silicone or other elastomeric materials. This creates devices that have better elastic properties, but with possibly less than ideal bio-compatibility.
Accordingly, without intending to limit or define the scope of the present invention with the following purposes, the present invention addresses deficiencies found in prior devices. First, it is a primary purpose of the present invention to create a tubular structure that exhibits limited leakage following puncture.
It is still another purpose of the present invention to provide a reduced-leakage tubular structure that incorporates reduced leakage properties with minimal increased space requirements.
It is a further purpose of the present invention to provide a reduced-leakage tubular structure that can be constructed substantially from highly biocompatible material, such as PTFE.
These and other purposes of the present invention will become evident from review of the following specification.